The spine is a flexible column formed of a plurality of bones called vertebrae. The vertebrae are hollow and piled one upon the other, forming a strong hollow column for support of the cranium and trunk. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies, or spinal discs. Various spinal disorders may cause the spine to become misaligned, curved, and/or twisted or result in fractured and/or compressed vertebrae. It is often necessary to surgically correct these spinal disorders.
The spine includes seven cervical (neck) vertebrae, twelve thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, and the fused vertebrae in the sacrum and coccyx that help to form the hip region. While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow shaft containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae.
The spinal discs act as shock absorbers, cushioning the spine, and preventing individual bones from contacting each other. Discs also help to hold the vertebrae together. The weight of the upper body is transferred through the spine to the hips and the legs. The spine is held upright through the work of the back muscles, which are attached to the vertebrae.
A number of approaches, systems, and apparatuses have been devised to accomplish a variety of surgical interventions in association with the spine. These approaches enable a surgeon to place instrumentation and implantable apparatuses related to discectomy, laminectomy, spinal fusion, vertebral body replacement and other procedures intended to address pathologies of the spine. The variety of surgical approaches to the spine have a number of advantages and drawbacks such that no one perfect approach exists. A surgeon often chooses one surgical approach to the spine from a multitude of options dependent on the relevant anatomy, pathology, and a comparison of the advantages and drawbacks of the variety of approaches relevant to a particular patient.
A common surgical approach to the spine is the lateral approach, which, in general, requires a surgeon to access the spine by creating a surgical pathway through the side of the patient's body through the psoas muscle to an intervertebral disc space where it is possible to dock onto the lateral lumbar disc. Variants of the lateral approach are commonly referred to as the “direct lateral” approach in association with the “DLIF” procedure, the “extreme lateral” approach in association with the “XLIF” procedure, and the “oblique lumbar” approach in association with the “OLIF” procedure.
A common problem associated with the lateral surgical approach includes a significant risk of damage to the musculature surrounding the spine. FIGS. 1A-1B illustrates a partial view of a spine 100 comprised of sequential vertebrae 109, each separated by intervertebral disc space 110, with an attached psoas muscle group 102 (including the psoas minor and psoas major). As shown, the psoas muscle 102 runs generally in a cranial-caudal direction with muscle fibers attached diagonally or at an approximate 45-degree angle to the spine 100. FIGS. 2A-2B illustrate an exemplary lateral approach to the spine. In typical lateral approaches, after making an incision in the psoas muscle 102, the surgeon places a number of sequential circular dilators 1041-n, each larger in diameter, on the desired pathway to the spine 100 through the psoas muscle 102 to dilate the surgical site radially away from the initial incision site or K-wire insertion point. This dilation process can lead to compression of muscle, nerves, and blood supplies adjacent to the vertebral body, which can lead to ipsilateral upper thigh pain, hip flexor weakness that causes difficulty in walking and/or stair climbing, and muscle atrophy that follows from muscle injury.
After the series of circular dilators are forced into the muscle tissue, a multi-bladed or tubular retractor apparatus 106 may be placed over the final dilator 104n. The retractor is then retracted radially to separate the psoas muscle and other soft tissues. A common problem associated with this type of lateral procedure is that soft tissues, including the musculature and nerves surrounding the spine, become crushed and/or trapped near the distal end of the retractor's blades when the retractor is passed over the final dilator, a problem often referred to as “trappage,” graphically depicted in FIG. 3.
In order for the surgeon to clear the surgical pathway to the disc space, or to “see” the disc space, the surgeon must cauterize and cut the muscle that is caught inside the retractor, effectively performing a muscle biopsy each time the surgeon performs an XLIF, DLIF, OLIF procedure. Beyond undesired muscular damage to the patient, this approach requires additional effort for the surgeon to utilize a cautery or similar device to remove the trapped soft tissues from between the distal end of the retractor and the vertebral bodies prior to completing access to the spine.
Oftentimes the resulting damage and trauma to the soft tissue resulting from trappage and removal of psoas muscle tissue with a cautery causes lasting problems for a patient. For instance, a patient who experiences trappage during surgery will often have ipsilateral upper thigh pain and leg weakness. Such pain and leg weakness occurs due to the linkage of the psoas to the lower body, as the psoas muscle connects to the femur. Thus, damage to the psoas will generally manifest in lower body discomfort, including pain and weakness in the leg.
Another problem associated with existing lateral surgical approaches to the spine is nerve damage. The lumbar plexus is a web of nerves (a nervous plexus) in the lumbar region of the body which forms part of the larger lumbosacral plexus. The lumbar plexus in particular is often damaged as a direct result of surgical intervention utilizing the lateral approach to the spine. The nerves associated with the lumbar plexus can experience indirect nerve injury as a result of over-dilation or over-retraction of apparatuses utilized to accomplish lateral access to the spine. They also can experience direct nerve injury as a result of direct trauma caused by impingement from the instrumentation utilized during the surgical intervention in association with the lateral approach to the spine, as in the case of trappage, discussed above. Such indirect and direct nerve damage can cause numbness in part or all of the leg and can lead to indirect muscle atrophy. A recent meta-analysis review of 24 published articles indicates that the lateral approach is associated with up to a 60.7% complication rate. Gemmel, Isaac D, et. al, Systemic Review of Thigh Symptoms after Lateral Transpsoas Interbody Fusion for Adult Patients with Degenerative Lumbar Spine Disease, International Journal of Spine Surgery 9:62 (2015). The review further found that the retractors resulted in 43% psoas muscle pain, 30.8% psoas muscle weakness, and 23.9% nerve or plexus injury due to the inherently flawed design of existing commercially available retractors.
One existing method of neuromonitoring involves the insertion of a number of epidural electrodes into the lumbar plexus. Stimulation of the electrodes is used to trigger a response in the patient's nerve structures, and the resulting evoked potentials correspond to the neural activities of the nerve structures near the recording electrodes. The potentials may be recorded to detect reactions in the nervous system that may indicate a problem, including some type of impingement or encroachment of an instrument upon the nerve structures during a procedure. This method, while providing information relating to a change in the behavior of the nerve structures nearby the inserted electrodes, does not directly correlate to a change in the behavior of the nerve structures in response to a nearby surgical instrument such as a dilator or a retractor, and is therefore not optimal for identifying impingement from the instrumentation utilized during the surgical intervention.
In addition, existing dilators oftentimes incorporate a vertical wire conductor that extends through the outer wall of the dilator parallel to the longitudinal axis of the apparatus, terminating in a pinpoint electrode at the distal end of the apparatus. The electrode may stimulate nearby nerve structures to asses for any impingement upon nerve or plexus. Because the vertical wire provides only a pinpoint electrode, the surgeon must manually rotate the apparatus through 360 degrees to perform a full range of neuromonitoring for impingement upon all of the adjacent neurological structures surrounding the device: the front and the back, superior and inferior. This additional step is cumbersome and presents challenges in achieving thorough neuromonitoring. Moreover, because existing dilators with pinpoint electrodes require the surgeon to rotate the dilators to achieve neuromonitoring in 360 degrees, the dilators cannot perform a full range of monitoring once they are affixed. After fixation, only pinpoint monitoring is provided, and existing devices cannot provide continuous, real-time neuromonitoring throughout the procedure.
Existing retractor systems also present challenges in terms of illumination and require a separate light source that attaches to the top of the retractor. This separate device is cumbersome, physically interfering and disruptive, and the limited ability to position the light source oftentimes means that light reflects off of the retractor blades before returning to the surgeons eyes, which leads to suboptimal visualization of the surgical area.
Existing retractor systems also lack ease of adjustability and are not designed with an eye toward ergonomic use by the surgeon, who is forced to hunch over the retractor apparatus during the course of the procedure to direct the surgical equipment as desired.